The present specification generally relates to methods for forming ceramic cores used in investment casting. Specifically, the present specification relates to methods for forming ceramic cores that have a silica depletion zone.
Investment casting often utilizes cores to produce internal channels inside cast metals. A molten metal or alloy is poured into a mold containing a core. After the metal solidifies, the core is removed to leave behind the internal channels. The architecture of the internal channels is determined by the features of the core.
Cores formed through injection molding and other conventional processes can produce simple hollow channel architectures. However, in some applications, such as cast blades for gas turbines, more complex channel geometries are desirable due to their improved blade performance, where air is blown through the hollow channels of the cast blade for cooling. Improved blade cooling performance can take the form of reduced cooling air flow, which allows for increased utilization of air for combustion and thus increases engine thrust. Higher blade cooling performance allows for an increase in combustor operating temperature and improved thermodynamic efficiency, resulting in better specific fuel consumption, while still maintaining turbine blade component temperatures within an acceptable range for durability. A major limitation to commercial implementation of these representative cooling circuits in turbine blades is the inability to produce the necessary ceramic cores as single piece articles by conventional molding techniques.
In some methods, disposable core die (DCD)s may be utilized to produce cores with both simple and complex channel architectures. In these methods, the core is formed by injecting a slurry containing ceramic particles, fugitive species, and an organic binder into a disposable core die. The slurry is subsequently cured and then fired to produce a solidified ceramic core. The disposable core is removed before, during, or after the core firing process, for instance by a chemical, thermal or mechanical process.
Ceramic core materials used in the investment casting industry are predominantly made of silica (SiO2). Silica is a commonly used core material in investment casting because of its high processability, low coefficient of thermal expansion, high-temperature dimensional stability, and ease of removal from the casting. Articles made by investment casting generally include cast metals or metal alloys. In some instances, alloying elements, such as aluminum, nickel, chromium, yttrium, zirconium, etc., used in the investment casting may react with the conventional silica-based ceramic core and cause deleterious effects, such as, for example, internal oxidation. The oxidation and loss of aluminum, nickel, chromium, yttrium and/or zirconium may cause rejection of an expensive casting. In addition, reaction between the alloying elements and the ceramic core may cause the ceramic core and the metal casting to tightly bond with each other and make the ceramic core more difficult to remove.
Yttrium addition to an alloy is known for improving oxidation resistance of nickel-based superalloys at the service temperature of turbine airfoils. However, yttrium can react with silica during casting, thereby resulting in depletion of the yttrium in the alloy and introduction of undesirable components into the alloy that debit the mechanical properties. Reaction between nickel-based superalloys with silica-based ceramic core is a major limitation in using silica cores for the casting of these (reactive) nickel-based superalloys. Alumina and yttria materials have been used in ceramic cores to reduce or eliminate this reactivity problem. Alumina, for instance, is less reactive with reactive elements such as, for example, yttrium, in comparison with silica. However, alumina requires higher processing temperatures for processing than silica-based materials. Therefore, it is desirable to have an improved ceramic core and a process for forming the same for investment casting of reactive metals and alloys.